Functionally graded materials (FGM) are advanced composite materials whose mechanical properties vary continuously from one surface to another at macro level. Thick walled annular structures are widely used in industry as nuclear reactors, long pipes used for carrying gases/oil, pressure vessels etc. Elastic analysis of FG thick-walled cylindrical and spherical pressure vessels subjected to internal pressure alone are studied. The material properties, Young' s modulus-Poisson' s ratio, are assumed to obey a graduation with the power-law function through the wall thickness. Under these assumptions, the governing equations of the vessels become a two-point boundary value problem. The analytical solution of such an equation cannot be obtained except for simple grading functions. Complementary Functions Method is performed to solve governing equation in order to obtain displacement and stress distributions depending on inhomogeneity parameters.